What is a coral?

Corals are actually tiny, soft-bodied animals that belong to the same group as jellyfish and sea anemones, the Cnidarians. An individual coral animal is called a polyp. Coral polyps can live alone, but typically polyps live in colonies that may consist of hundreds to hundreds of thousands of individuals.

Corals are split into two main groups: soft corals and hard corals. Hard corals, also known as reef building corals, are the best-known type of coral, forming the large, colourful reefs found in clear, tropical seas. There are over 800 known species of reef-building coral.

How do coral reefs form?

The polyps of reef-building corals have a hard, protective calcium carbonate (limestone) skeleton, which attaches to a rock on the sea floor. Coral polyps can divide, or bud, into thousands of clones which are genetically identical to each other. Reefs begin to form as the hard skeletons of the living and dead polyps in a coral colony cement together.

As the coral colony grows over hundreds and thousands of years, it joins with other colonies and become a reef. Tropical reefs can grow at rates of 1 to 100 centimetres per year and can form huge structures over incredibly long periods of time, making them the largest and oldest living systems on Earth. Some of the coral reefs that exist today began growing over 50 million years ago.

How do corals get their food?

The polyps of reef-building corals have a special symbiotic relationship with single-celled algae, called zooxanthellae, which benefits both the algae and the coral. The zooxanthellae live inside the tissues of the coral and capture energy from the sun through photosynthesis, providing the coral with food. Up to 90% of a coral’s nutrients are obtained from the algae. In return, the coral provides the algae with protection, access to sunlight, and nutrients from plankton that is captured by the polyp.

Like jellyfish and sea anemones, coral polyps also have stinging tentacles that can they use to catch food. Corals mainly feed at night, extending the tentacles into the water current to catch plankton, small food particles and sometimes even small fish.

Where are coral reefs found?

Corals need salt water to survive and are found throughout the oceans, from deep, cold waters to shallow, tropical seas.

The most familiar and well-known corals, the reef-building corals, come from tropical regions. They grow best in warm, clear, shallow waters where lots of sunlight is able to filter through the water column to reach the symbiotic algae that live within their tissues.

Why are coral reefs important for biodiversity?

Coral reefs are among the most valuable ecosystems on Earth, forming massive, complex habitats for thousands of other species. Although coral reefs cover less than 1% of the ocean floor, the numbers of species living in, on and around coral reefs rivals even the most diverse tropical forests, including the Amazon rainforest in Brazil. Home to an estimated 2 million species, coral reefs are sometimes nicknamed the ‘rainforests of the oceans' because of their staggeringly rich biodiversity.

As well as supporting an enormous variety of species, tropical coral reef ecosystems are also important to human health and well-being. Coral reefs are vital to the world’s fisheries, forming nurseries for about a quarter of the ocean's fish and providing a source of income for local communities, as well as national and international fishing companies.

In coastal areas, coral reefs provide an important natural barrier against waves, hurricanes, typhoons and even tsunamis, helping to prevent coastal erosion and flooding. The beauty and diversity of coral reefs make them important tourist attractions which help to generate income and create jobs for local communities. Coral reefs are also becoming increasingly important in medicine, and coral reef organisms are already being used in treatments for diseases like cancer and HIV.

Why are coral reefs under threat?

Coral reefs are becoming increasingly threatened, and scientists estimate that the threats to these delicate ecosystems, which include climate change, pollution, overfishing and sedimentation, could result in the loss of a third of the world’s remaining coral reefs within the next 30 years.

Climate change has been identified as one of the greatest global threats to coral reefs. Rising sea temperatures mean that a process called coral bleaching, where the corals lose their symbiotic zooxanthellae because of the stress of being exposed to extreme temperatures, is becoming much more frequent. Coral bleaching events cause many corals to die as they can no longer obtain enough nutrients and begin to starve. Coral reefs that suffer bleaching can take years or even decades to recover. Climate change also causes something called ocean acidification, as more carbon dioxide from the atmosphere is absorbed into the ocean, altering the water chemistry. As the oceans become more acidic, it reduces the ability of corals to build their limestone skeletons.

Coral conservation

Coral reef ecosystems are threatened around the world. As a result, many marine conservation programmes have been developed by conservation organisations which aim to study and conserve coral reefs. Some of these conservation organisations work locally, while other organisations have developed global initiatives such as Reef Check and the International Coral Reef Initiative, allowing scientists to share and compare the data they collect.

Coral conservation is complex; integrating many different conservation management approaches which consider all aspects of coral ecosystems, as well as the needs of the people and communities that rely on their resources. Education, outreach and community engagement are often a key component of conservation programmes and can help to reduce a number of threats, such as over-fishing and land-based pollution.

Marine Protected Areas (MPAs) are one of the most effective ways to protect and restore coral reef ecosystems, especially when they are used in combination with additional conservation and management strategies.

Research and monitoring of coral reefs is also vitally important in ensuring that the world’s reefs remain healthy. Initiatives such as the NOAA Coral Reef Watch (CRW) use satellite data on sea surface temperature to alert scientists, conservation managers and decision-makers around the work to the risk of coral bleaching, and are even using new technologies to predict future coral bleaching events before they occur.

Real science: Team WILD

Why do Root and Flora, our Team WILD science superheroes, need help surveying coral reefs in the Chagos Archipelago? Why do scientists survey coral reefs and how do they do it?

Coral reef conservation – Chagos case study

The Chagos Marine Reserve is the world’s largest marine reserve, home to some of the healthiest reef ecosystems and cleanest waters in the world. The spectacularly vibrant coral reefs of Chagos are home to an astounding variety of marine life.

Chagos is one of the few marine ecosystems where human activities haven’t yet had a big impact, making it important for global scientific research that aims to help our understanding of climate change, tropical marine ecosystems and the impacts of commercial fisheries.

Where is Chagos?

Chagos is an archipelago of 55 tiny islands in the central Indian Ocean and spans thousands of miles of remote tropical ocean. The Chagos Islands and the waters that surround them are a UK Overseas Territory, also known as the British Indian Ocean Territory (BIOT).

Why is Chagos important?

The Chagos Archipelago is an exceptionally diverse ecosystem, supporting an exceptional diversity of species and a vast number of different habitats. Chagos has been desgnated as the world’s largest no-take Marine Protected Area (which means that no commercial fishing is allowed), providing a safe haven for the region’s rich marine life.

Marine Protected Areas, such as the Chagos Marine Reserve, provide species with vital protection from human exploitation and help to restore marine ecosystems and the species they support. In Chagos, this includes many threatened species such as green turtles, hawksbill turtles and a range of shark species, along with globally important populations of seabirds.

Coral conservation in Chagos

The coral reefs in Chagos are some of the healthiest and most resilient reef systems in the world. Chagos is one of the few marine reserves where humans haven’t yet had a very big impact on the environment, and because the coral reefs there are in such good condition they are an important benchmark for coral conservation.

The waters around Chagos support over 220 coral species and account for almost half of the recorded species of the entire Indian Ocean, ranging from shallow water brain corals, such as the endemic brain coral, to staghorn corals and a variety of other species that flourish in the deepwater habitats.

Scientists working in Chagos can study the complex, diverse and relatively pristine reef ecosystems and compare the effects that major threats to coral reefs, such as climate change and the impacts of commercial fisheries, are having on the reefs in Chagos compared with the effects that the same threats are having on less healthy or less well protected reefs in other parts of the world.

How and why do scientists survey corals reefs?

There are many different ways for scientists to survey coral reefs and the different types of species that are found there. There are also many different reasons why scientists need to survey coral reefs, from simply collecting data on the different kinds of species that live on the reef, to monitoring the health or recovery of the reef and the effects that climate change, fishing and other factors are having on the whole coral reef ecosystem.

Scientists have many different ways of carrying out these surveys, but most coral reef surveys involve undertaking transects (surveys along a straight line) at various depths across the reef in order to gather detailed information from different habitats. Divers will swim along transects in a given direction and will survey the reef at different points, marking each point using a GPS so that the transect can be repeated to gather data over time.

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